Device for correcting the pitch of lead screws in screw-cutting machines



United States Patent O "ice 3,130,617 DEVICE FOR CORRECTING Tim PICI-I F LEAD SCREWS IN SCREW-CUTTING MACHINES Pierre Franois de Vailiere, 14 Avenue de Bretteville, Neuiily-sur-Seine, France Filed Nov. 21, 1964), Ser. No. 70,725 Claims priority, application France Nov. 20, 1959 1 Claim. (Cl. 82-5) This invention relates to improved devices for correcting the pitch of lead screws in screw cutting machines.

In a screw-cutting machine, of the lathe or grinder type in which very high precision screw threads are to be formed, it is customary to incorporate into the ma.- chine a compensation cam which, by its action on the lead-screw nut, makes such pitch corrections as may be necessitated by imperfections originally existing in the lead-screw or which arise from wear, and especially irregular wear of the lead screw.

Upkeep of this cam and such corrections thereto have heretofore been obtained by laborious checks carried out with screw or standard jig systems.

It is one of the main objects of the present invention to overcome the need of such systems which constitute severe disadvantages.

To this end an embodiment of the invention comprises a correcting device consisting of a sighting device which is mounted on a machine-tool carriage and in which can be visually ascertained, any error existing between said carriage driven by the lead screw alone-in a position which may or may not be corrected by a cam of the type referred to-and a spiral mark in the form of a reference line carried by a support which is rotated together with the lead screw.

The sighting device preferably consists of an optical system provided with a collimator, or of a microscope of similar nature.

In one embodiment of such a mechanism mounted on a screw-cutting machine and comprising such an optical system rigidly locked to the tool carriage, the mark in question preferably consists of a scored line on a polished steel cylinder rotated together with the lead screw. The rotation of the cylinder preferably is at the same speed as the lead screw although a speed reduction or multiplication system may be employed as desired.

In a simplified embodiment, the reference line is scored directly on the heads of the lead screw threads, since these heads are not subjected to wear or distortion.

Provision is thereby made for a machine with a built-in compensation system, that can be checked at all times and at any point along the travel of the carriage.

Such a system permits verications to be made at all times of the accuracy of the compensation cam profile, thereby enabling any defects in it to be corrected.

Moreover, in order to dispense with the need of carrying out costly cam truing-up operations which can only be performed by metal-removing grindings or by using extra ller metal, the invention further contemplates a device which comprises a compensation cam of adjustable profile constituted by the juxtaposition of a plurality of resilient blades which are ixed, from place to place, to supports which are themselves adjustable relative to a reference bar.

The adjustable supports preferably consist of a set of yokes which retain the outer cam blade and whose central space is occupied by a threaded adjustment pin which compresses said blade and which is screwed into a reference bar and thrusts, via its head, against an adjustable element at the bottom of the corresponding yoke and, via its tip, beneath said juxtaposed blades.

Such an assembly bears a certain resemblance to the lhl? Patented Apr. 28, 1964 adjustable moulds or curves used by draughtsmen for drawing curves.

The description which follows with reference to the accompanying drawing is given by Way of example only and not in a limiting sense. In the drawing:

FIG. l is a diagrammatic view in elevation of a correcting device in accordance with the invention.

FIG. 2 is a diagrammatic proiile view of FIG. l.

FIG. 3 is a view of the side of the compensation cam.

FIG. 4 is a corresponding proiile view of the cam in FIG. 3.

A lathe carriage 1 is driven by a nut 2 cooperating with a lead screw 3, said lead screw being coupled, via a train of gears comprising pinions 4, 5 and 6, to the machine spindle 7 carrying the workpiece. The pinion 4 meshes with a pinion 8 rigidly connected to a polished metal cylinder 9. On this cylinder 9 there is scored a reference spiral line 16. The carriage 1 is provided with a support 11 onto which is mounted a sighting device 12 provided with a collimator.

The carriage nut 2 is mounted without endwise play on the carriage, but with rotational play arrested by stops. The nut 2 is provided with a finger 13 terminating in a feeler 14, and this feeler is caused to follow the prole of a cam 15 extending in substantially parallel fashion to the axis of the lead screw 3.

The tool 1a carried by the turret 1b on the carriage 1 cuts a screw-thread on the workpiece (not shown), and the position of this tool can be checked at all times by taking a sighting of the line 1d through the device 12. Any positional error of the tool shows up immediately in the collimator.

Any error which may become apparent can only be caused by wear in the lead-screw 3 not compensated for by the proiile of the cam 15. In this way, the degree of wear can be kept under constant check and any camtruing operations made can subsequently be verified.

Under marginal conditions, the work could be followed continuously by an observer thereby eliminating the correcting control system acting on the nut 2. Since there is no material link between the carriage and the line 16, the cylinder Si sustains no wear and the inherent precision of the check remains unaiiected. Evidently, the optical control system could be replaced by an observer, and such a system would comprise a mechanism which would cause the position of the carriage to follow the position of optical alignment between the reticle of the viewing device, and the reference line.

In practice, a highly satisfactory degree of precision can be obtained by the action of a cam such as the cam 15, which is capable of verication at all times.

In order to avoid the need to carry out dismantling, grinding or ller-metal-applying operations When truing up the cam, such a cam 15 may be constituted by the superposed resilient metal blades 16 (see FIGS. 3 and 4). At one end, a blade 16a serves as a thrust surface for the feeler 14. This blade 16a is tted beneath the toes 17 of a yoke 18 set astride a reference bar 19 lixed parallel to the lead-screw 3. The inner blade leb has exerted against it, in a direction opposite to the thrust of the toes 17, the thrusting action of a threaded pin 20 contained between the arms of the yoke and screwed into a corresponding hole in the bar 19. The head 21 of rod 1S is provided with orices 22 designed for cooperation with a tommy-bar and thrusts against the end of a setscrew 23 which is threaded into hole 24 provided in the bottom of the yoke 25. With such means, the proile of the blade 166 can be modied as required by distributing an appropriate number of yokes 18 along the bar 19 at suitable points which may or may not be evenly spaced.

In such an assembly, the viewing device 12 may be,

for example, a microscope incorporating an internal error-measurement scale, or a microscope mounted on a support with micrometric sliders. The observed errors, which may be obtained either by direct ratio or by conversion with the aid of a preestahlished numerical table, provide an evaluation of the corrections to be made to the cam prole.

Should it be desired to dispense with the engraved cylinder and its train of gears, the reference line in question can be provided on the heads of the lead screw threads themselves. However, an engraved cylinder with a reducing or multiplying train of gears can prove advantageous in certain cases, particularly to enable a single engraved cylinder to be used with lead screws of different pitches merely by replacement of the pinions, in order to insure coincidence at all times between the apparent pitch of the engraved line and the true pitches of the lead screws.

lt is to be clearly understood that many modifications can be made to the embodiments described hereinbefore without departing from the scope or the invention. Thus such a machine can be equally suited to cutting-tools or to truing grinders; moreover, the error-reading device can permit corrections to be made by an arrangement other than one utilizing a nut and correction-cam, for example, by an arrangement providing for axial or angular displacement of the lead screw itself, with or without an interposed automatic follow-up device.

d What I claim is: In a lead screw pitch correcting device on a screw cutting machine, provided with a tool holding carriage, a lead screw displacing said carriage and a lead screw wear compensator, comprising, in combination, a microscope with collimator iixed on said carriage, a support rotatably driven by said lead screw, a reference spiral line traced on said support, viewable through said microscope, and a wear compensator for compensating for possible irregularities of the threads of said lead screw, said compensator including at least one elastically deformable element, a reference support on said machine, holding members with screws and yokes between said reference support and said elastically deformable element, a nut pivotally mounted without axial play on said carriage and engaging said lead screw, and a lever arm fixed on said nut and following said elastically deformable element.

References Cited in the le of this patent- UNITED STATES PATENTS 301,165 Rogers et al. July l, 1884 1,347,789 Muller July 27, 1920 2,169,159 Moller Aug. 8, 1939 2,811,685 Burke Oct. 29, 1957 2,916,931 Cunningham Dec. 15, 1959 FOREIGN PATENTS 665,582 Great Britain Jan. 23, 1952 

